Filtration is a process by which particles may be separated from a fluid or mixture of particles by taking advantage of differences in their physical properties. It occurs when a fluid is passed along or across a membrane or other structure that can act as a selective barrier. Particles are either retained by the barrier or pass through in the filtrate.
Since their invention in the 1960s track-etched polymer filters have been widely used for biological cell enrichment because of their low cost and fast sample processing speed. The track-etched filters have randomly distributed pores defined by swift heavy ions typically generated by nuclear reactors and then enlarged to a desired diameter in an etching process. These filters mechanically enrich certain cells from body fluids based on their ability to pass through pores of a particular size.